A stand-alone microgrid is a small-scale power supply system composed of a wind power generator, a solar power generator, an inverter, a battery, a diesel generator, etc. The main different between the stand-alone microgrid and a conventional power station is that which equipment maintains a voltage and a frequency of an electric power system. In the conventional power station, a power generator maintains the voltage and the frequency of the electric power system, and in the stand-alone microgrid, the inverter maintains the voltage and the frequency.
In the small-scale power supply system, electric power system impedance is relatively large, compared to a large electric power system, and thus depending on the type of the load, a voltage waveform is easily distorted or voltage inequality often occurs. Also, since the inverter of the stand-alone microgrid does not take this phenomenon into consideration, voltage quality is degraded or is not enhanced.
The inverter of the stand-alone microgrid controls the voltage and the frequency to be constantly maintained regardless of the magnitude and type of the supplied load. Here, when the generated voltage waveform is similar to a complete sine wave, voltage quality is good. To this end, conventionally, a voltage controller of the inverter is used as a synchronous reference frame proportional-integral voltage controller (hereinafter, referred to as a PI controller). Here, the PI (Proportional and Integral) controller is simple in structure and is easily realized, and when supplying a three-phase balanced load, the result of synchronous reference frame conversion of a three-phase voltage is a DC component, and thus voltage control (a voltage waveform similar to a sine wave) without an error is possible. However, in the PI controller, when the non-linear load or the unbalanced load is supplied, an unbalanced and harmonic component occurs between three-phase voltages, and a synchronous reference frame d-q axis voltage contains a ripple component, and thus a steady-state error of the voltage can be eliminated. That is, the voltage supplied to the load is distorted, which may cause damage and malfunction in an important load.
In this regard, there is Korean Patent Application Publication No. 10-2010-0102357, titled “a current control apparatus of a distribution static compensator considering a non-linear load.”